Heat treatment of nickel-chromium-cobalt base alloys

ABSTRACT

A four-stage heat-treatment to improve the stress-rupture life and stress-rupture ductility at elevated temperatures of nickelchromium alloys in which a two-stage heat-treatment is used intermediate of an initial solution treatment and a final aging treatment.

United States Patent 1 1 Shaw [ 1 Aug. 5, 1975 1 1 HEAT TREATMENT OFNlCKEL-CHROMlUM-COBALT BASE ALLOYS [75] lnventor: Stuart Walter KerShaw, Wylde Green, England [73] Assignee: The International NickelCompany,

Inc., New York,

22 Filed: Jan. 2, 1974 21 Appl.No.: 430,111

[30] Foreign Application Priority Data Sept. 6, 1973 United Kingdom41888/73 [52] US. Cl. 148/162; 75/134 F; 75/171; 148/13; 148/32.5 [51]Int. Cl. C22c 19/00; C22f 1/10 [58] Field of Search 75/134 F, 171;148/13, 148/32, 32.5, 162

[56] References Cited UNITED STATES PATENTS 2,766,155 10/1956 Betteridgeet a1. 148/162 3,356,542 12/1967 Smith 148/162 3,459,545 8/1969 Bieberet a1. 75/171 3,479,157 11/1969 Richards et al. 75/171 3,536,542 10/1970Murphy et a1. 148/162 3,653,987 4/1972 Boesch 148/162 3,667,938 6/1972Boesch 148/162 3,723,107 3/1973 Richards 148/162 3,741,821 6/1973 Atheyet a1. 148/162 3,748,192 7/1973 Boesch 148/162 FOREIGN PATENTS ORAPPLICATIONS 731,441 6/1955 United Kingdom 75/162 715,140 9/1954 UnitedKingdom..... 75/162 777,703 6/1957 United Kingdom..... 75/162 1,053,10912/1966 United Kingdom..... 75/162 1,196,714 7/1970 United Kingdom75/162 1,333,354 10/1973 United Kingdom 75/162 Primary Examiner-C.Lovell Attorney, Agent, or Firm-Raymond J. Kenny; Ewan C. MacQueen 57ABSTRACT A four-stage heat-treatment to improve the stressrupture lifeand stress-rupture ductility at elevated temperatures of nickel-chromiumalloys in which a two-stage heat-treatment is used intermediate of aninitial solution treatment and a final aging treatment.

8 Claims, N0 Drawings HEAT TREATMENT OF NICKEL-CHROMIUM-COBALT BASEALLOYS The instant invention relates to the heat-treatment ofnickel-chromium-cobalt base high-temperature alloys.

In my United States application No. 241,443, now abandoned, there isdescribed and claimed castable nickel-chromium-cobalt alloys containingabout 0.02 to 0.25 percent carbon, about 20 to 25 percent chromium,about 5 to 25 percent cobalt, one or both of molybdenum up to 3.5percent and tungsten up to 5 percent in such amounts that the value 0.5percent Mo) plus %W is from 0.5 to 5 percent, about 1.5 to 5 percenttitanium and l to 5 percent aluminium, with the provisos that the sum ofthe aluminium and titanium (i) be from 4 to 7 percent and (ii) intungsten-free alloys not exceed 6 percent, and further that the (iii)ratio of titanium to aluminium be from 0.75:1 to 4:1, about 0.5 to 3percent tantalum, up to 3 percent niobium, about 0.005 to 1 percentzirconium and up to 2 percent hafnium, with the proviso that the valueof percent Zr 0.5 (percent Hf) be from 0.01 to 1 percent, about 0.001 to0.05 percent boron, and up to 0.2 percent in total of yttrium orlanthanum or both, the balance, apart from impurities, being nickel inan amount of at least 30 percent.

In order to develop a high level of stress-rupture strength at elevatedtemperatures such as 927C (1700F) or 980C (1,800F), the alloys were tobe subjected, upon casting, to a heat-treatment comprised ofsolution-heating and subsequent aging. Various heat-treatments of thisgeneral kind were given in my said US. Application, namely:

a. Solution-heating at 1,0501250C for 1-10 hours;

aging at 600950C for l-24 hours.

b. Solution-heating at 1,0501,250C for 1-20 hours; intermediate heatingat 8001,150C for l-16 hours; aging at 600950C for 1-24 hours.

c. Solution-heating at 1,200C for 16 hours; air-cool; heat at1,1001,150C for 2-4 hours; air-cool; age at 800C for 16 hours; air-cool.

However, for many purposes, eg for gas turbine noz zle guide vanes,rotor blades and integrally bladed rotors, for small gas turbines suchas for automotive propulsion, auxiliary engines on aircraft or ships andfor diesel engine turbo-blowers, it is extremely important that articlesand parts cast from high-temperature alloys should not only have highstress-rupture strength at the aforementioned high temperatures of 927Cor 980C, but also have a combination of high stressrupture strength andstress-rupture ductility (as measured by the elongation at fracture) atintermediate temperatures, e.g. 816C (1500F).

Given the emphasis on stress-rupture strength, ob taining an improvedlevel of ductility has not been without difficulty. For it will beappreciated that the ductility of alloys of a given base composition ata given temperature generally decreases as the stress-rupture strengthis increased by minor modifications in composition. And the results inTable ll of my prior Application reflect that the stronger alloys, i.e.,those for which /2 (percent Ta) percent Nb percent Ti percent Al isbetween 6.7 and 7.7, generally have elongations at rupture, when testedunder a stress of 276 l\l/mm (28 kgf/mm at 816C, of less than about 4percent. The specimens had been heat-treated by solutionheating at1,150C for 4 hours, air-cooled, aged at 850C and again air-cooled.Little improvement in duetility resulted from the interposition of anintermediate heating in between the solution-heating and aging, as inheat-treatment (b) above.

The present invention is based on the discovery that the stress-ruptureductility and the stress-rupture life at 816C for the alloysaforedescribed can be substantially improved provided there isinterposed between the solution-heating and aging steps a two-stageintermediate treatment at temperatures within critical ranges.

According to the invention the alloys are given the followingheat-treatment:

i. solution-heating at 1 1 C for from 2 to 16 hours (preferably at about1150C for about 4 hours) ii. heating at-970l030C for from 2 to 10 hours(preferably at about 1,000C for about 6 hours) iii. heating at 870930Cfor from 8 to 48 hours (preferably at about 900C for about 24 hours) iv.aging at 600-800C for from 8 to 48 hours (preferably at about 700C forabout 16 hours) The alloys may be cooled at any convenient rate aftereach heat-treatment stage, e.g. by air-cooling generally toroom-temperature).

The improvement brought about by this four-stage heat-treatment is mostsurprising, particularly since 1 have determined that two prior artfour-stage heattreatments (there could be others) fornickelchromium-cobalt base alloys seriously impair either thestress-rupture life or ductility at 816C.

It is well known that if the more advanced nickelchromium base alloyswhich possess the highest creeprupture strengths attainable at aparticular chromium content contain a small excess of hardeningelements, sigma phase can be formed during very long time service understress at temperatures in the range of about 800 to 870C. This is alsotrue for alloys of the present invention, and for such very prolongedservice in this temperature range in order to eliminate the possibilityof sigma phase formation (or render the amount that could be formed sosmall that its effect on the properties of the alloys would benegligible) (i) the value of /2 (percent Ta) percent Nb percent Tipercent Al should not exceed 7.7 percent, (ii) the chromium contentshould not exceed about 23.5 percent and (iii) the W /2 (percent Mo)should not be greater than about 2.5 percent.

The advantages of the heat-treatment of the invention are clearly shownby the results of tests on an alloy of the same nominal composition asAlloy No. 24 of my prior Application, namely:

23 percent Cr, 15 percent Co, 2 percent W, 1 percent Nb, 1.4 percent Ta,3.5 percent Ti, 1.9 percent A1, 0. 15 percent C, 0.1 percent Zr, 0.01percent B, balance (apart from impurities) Ni.

Test-pieces machined from tapered test-bar blanks cast from avacuum-melted heat of this alloy were given various heat -treatments andthen subjected to stressrupture tests at 816C under a stress of 276 N/mmand to tensile tests at 20C. The heat-treatments and tests results areset forth in Table l.

TABLE I Strcssrupturc Tensile C Heat Treatment 27(1/81(iC U.T.S.

A 411 1 150C 6h/l()()0C +24h/900C lob/700C 1572 8.8 941 3.7 B 411/1 150C2411 900c 1290 5.2 897 1.0 +16h/7()0C 1327 5.8 903 0.8 C 4h/l 150C 6h1000C +l6h/70()C 1358 4.4 942 2.2 D 4h/1 150C 16h/850C 1336 3.4 1010 4.31399 4.0 1020 2.9 E 411/1 163C 411/ 1080C +24h/927C l6h/760C 430 8.5 9221.1 F 411/1 177C 4l1/l080C +24h/843C loll/760C 907 2.6 1030 1.2

U.T,S. Ultimate Tensile Stress In each case the specimens wereair-cooled to roomnominal composition: 22.7 percent Cr, 19 percent Co,temperature between successive stages of the heat- 2 percent W, 1percent Nb, 1.4 percent Ta, 3.7 percent treatment. Ti, 1.9 percent Al,0.15 percent C, 0.1 percent Zr, 0.01 Of the six heat-treatments in TableI, only Treatment 20 percent B, Ni balance (apart from impurities). ThisA is in accordance with the invention. In each of Treatll i stronger istress-rupture than the alloy used in ments B and C On f th tintermediate heating the tests reported in Table l, and accordingly thestresssteps was mitted, th s being examples of the rupture tests at 816Cwere carried out under the Stage heat-treatment of y Prlor pp higherstress of 310 N/mm The results of stress- Tfeatmem D represents a PrlorPreferred twostage rupture tests at 927C under a stress of 120 N/mm aretreatment. E and F are two prior art fourstage treatalso given mentsdescribed in U.K. specification No. 1,248,492.

TABLE I1 Stress-rupture Tensile 20C Heat Treatment 70 U.T.S.

N/mm C Life h. Elong. N/mm Elong.

A 411/1 150C 6h/l000C 310 816 1101 9.4 950 3.0 +24h/900C Sh/700C 120 927800 8.0 965 3.8 1 D 4h/1150C+ lob/850C 310 816 915 2.8 846 0.5 120 927817 8.1 875 0.7

It will be seen that Treatment A gives the best combi- These datareflect that the stress-rupture life and nation of rupture life andductility and tensile ductility, 40 d ilit t 816C and the tensileductility at roomthe stress'rupture ductility being more than twicfithat temperature are again substantially increased by Treatgiven -by thetw -Stag Tr a D with the Stfe$5 ment A compared with Treatment D, whilethe stressrupture life being some 20 percent greater. Treatments ruptureproperties t 927C are substantially un- B and C both afford someimprovement in stressh d rupture ductility, but Offer no signififlamChange It is thought that the heat-treatment according to thestress-rupture life, and both treatments impaired tensile 'lnventlon mayalso advantageously be applled to the dummy Pompared Treatment alloys inthe wrought form. Moreover, it is considered The prior art four-stageTreatments and F bgth that the special sequence of heating steps hereinderiously detracted from stress-rupture life at 816 C and scribed isapplicable to other nlckel chromlum high I i a ll f t compared z j g A(211805)) temperature superalloys, particularly those hardenable d It 39J fi L fz stress'm? ure by at least from 3 percent to 12 percent oftitanium plus uctl i is was assoc? 6 W1 very poor 8 ress' aluminium,and/or matrix hardenable by one or more rurture l f h l n m l t d hereinof niobium, tantalum, molybdenum and tungsten, with n terms 0 morp o aOys co emp a 6 elements such as cobalt, zirconium, boron, carbon andupon being subjected to the four-stage sequence of heating have astructure characterized by the presence in the grain boundaries ofsmall, discrete M C -type carbide particles without significant extraprecipitation or denudation of gamma prime precipitates in adjacentareas.

particularly good reSults are obtained with alloys consistingessentially of about 0.02 to about 0.25 carhaving compositions withinthe following ranges: 22.2 about 20 to about 25 Percent Chromium, about5 to 22 8 percent C t 19,5 percent Co 8 t 22 percent to about 25 percentcobalt, at least one metal percent W, 0.9 to 1.1 percent Nb, 1.3 to 1.5percent from the group of molybdenum up to 3.5 percent and Ta, 3.6 to3.8 percent Ti, 1.8 to 2.0 percent Al, 0.13 to 6 tungsten up to 5percent in amounts such that 0.5 per- 0.17 percent C, 0.04 to 0.12percent Zr, 0.004 to 0.012 ccnt Mo plus percent W is from 0.5 to 5percent, about percent B, balance Ni. In Table ll are set forth the rc-1.5 to about 5 percent titanium, about 1 to about 5 per- Sults of testson an alloy within this range having the cent aluminium, the sum of theTi and A] being from hafnium being optionally present.

1 claim:

1. A heat treating process for improving properties of nickel-chromiumalloys at elevated temperatures on the order of '8 1 5C which comprisessubjecting an alloy about 4 to about 7 percent and not more than about 6percent in tungsten free alloys, and the ratio of titanium to aluminiumbeing about 0.75:l::4:l, about 0.5 to 3 percent tantalum, up to 3percent niobium, about 0.005 to 1 percent zirconium, up to 2 percenthafnium, the sum of the Zr 0.5 Hf being from 0.0l to l percent about0.001 to 0.05% boron, the balance being essentially nickel, the nickelbeing at least about 30 percent, to the following sequence of heattreating operations:

a. heating at about 1,1 20 to about 1, l 80C for about 2 to about 16hours; b. heating at about 970 to about 1.030C for about 2 to abouthours; c. heating at about 870 to 930for about 8 to about 48 hours; andd. heating at about 600 to about 800C for about 8 to about 48 hours. 2.The process in claim 1 in which heating (a) is for about 4 hours atabout ll50C.

3. The process in claim 1 in which heating (b) is for about 6 hours atabout 1000C.

4. The process in claim 1 in which heating (c) is for about 24 hours atabout 900C.

5. The process in claim 1 in which heating (d) is for about l6 hours atabout 700C.

6. The process in claim 1 in which the alloy treated contains about 0.13to about 0.17 percent carbon, about 22.2 to about 22.8 percent chromium,about 18.5 to about 19.5 percent cobalt, about 1.8 to about 2.2 percenttungsten, about 1.3 to about 1.5 percent tantalum, about 0.9 to about1.1 percent niobium, about 3.6 to about 3.8 percent titanium, about 1.8to 2 percent aluminium, about 0.04 to 0.12 percent zirconium, about0.004 to 0.012 percent boron, and the balance essentially nickel.

7. The process in claim 1 in which a. /2 percent Ta percent Nb percentTi percent Al is not greater than 7.7 percent;

b. the chromium content does not exceed about 23.5

percent; and

c. /2 percent Mo percent W is not greater than about 2.5 percent.

8. The alloy produced by the process of claim I having a structurecharacterized by the presence in the grain boundaries of small, discreteM C -type carbide particles without significant extra precipitation ofgamma prime precipitate in adjacent areas.

1. A HEAT TREATING PROCESS FOR IMPROVING PROPERTIES OF NICKEL-CHROMIUMALLOYS AT ELEVATED TEMPERATURES ON THE ORDER OF 815*C WHICH COMPRISESSUBJECTING AN ALLOY CONSISTING ESSENTIALLY OF ABOUT 0.02 TO ABOUT 0.25CARBON, ABOUT 20 TO ABOUT 25 PERCENT CHROMIUM, ABOUT 5 PERCENT TO ABOUT25 PERCENT COBALT, AT LEAST ONE METAL FROM THE GROUP OF MOLYBDENUM UP TO3.5 PERCENT AND TUNGSTEN UP TO 5 PERCENT IN AMOUNTS SUCH THAT 0.5PERCENT MO PLUS PERCENT W IS FROM 0.5 TO 5 PERCENT, ABOUT 1.5 TO ABOUT 5PERCENT TITANIUM, ABOUT 1 TO ABOUT 5 PERCENT ALUMINIUM, THE SUM OF THETI AND AL BEING FROM ABOUT 4 TO ABOUT 7 PERCENT AND NOT MORE THAN ABOUT6 PERCENT IN TUNGSTEN FREE ALLOYS, AND THE RATIO OF TITANIUM TOALUMINIUM BEING ABOUT 0.75:1::4:1, ABOUT 0.5 TO 3 PERCENT TANTALUM, UPTO 3 PERCENT NIOBIUM, ABOUT 0.005 TO 1 PERCENT ZIRCONIUM, UP TO 2PERCENT HAFNIUM, THE SUM OF THE ZR+0.5 % HF BEING FROM 0.01 TO 1 PERCENTABOUT 0.001 TO 0.05% BORON, THE BALANCE BEING ESSENTIALLY NICKEL, THENICKEL BEING AT LEAST ABOUT 30 PERCENT, TO THE FOLLOWING SEQUENCE OFHEAT TREATING OPERATION: A. HEATING AT ABOUT 1,120* TO ABOUT 1,180*C FORABOUT 2 TO ABOUT 16 HOURS, B. HEATING AT ABOUT 970* TO ABOUT 1.030*C FORABOUT 2 TO ABOUT 10 HOURS, C. HEATING AT ABOUT 870* TO 930* FOR ABOUT 8TO ABOUT 48 HOURS, AND D. HEATING AT ABOUT 600 TO ABOUT 800*C FOR ABOUT8 TO ABOUT 48 HOURS.
 2. The process in claim 1 in which heating (a) isfor about 4 hours at about 1150*C.
 3. The process in claim 1 in whichheating (b) is for about 6 hours at about 1000*C.
 4. The process inclaim 1 in which heating (c) is for about 24 hours at about 900*C. 5.The process in claim 1 in which heating (d) is for about 16 hours atabout 700*C.
 6. The process in claim 1 in which the alloy treatedcontains about 0.13 to about 0.17 percent carbon, about 22.2 to about22.8 percent chromium, about 18.5 to about 19.5 percent cobalt, about1.8 to about 2.2 percent tungsten, about 1.3 to about 1.5 percenttantalum, about 0.9 to about 1.1 percent niobium, about 3.6 to about 3.8percent titanium, about 1.8 to 2 percent aluminium, about 0.04 to 0.12percent zirconium, about 0.004 to 0.012 percent boron, and the balanceessentially nickel.
 7. The process in claim 1 in which a. 1/2 percentTa + percent Nb + percent Ti + percent Al is not greater than 7.7percent; b. the chromium content does not exceed about 23.5 percent; andc. 1/2 percent Mo + percent W is not greater than about 2.5 percent. 8.The alloy produced by the process of claim 1 having a structurecharacterized by the presence in the grain boundaries of small, discreteM23C6-type carbide particles without significant extra precipitation ofgamma prime precipitate in adjacent areas.